1. Field of the Invention
The present invention relates to an exhaust gas purifier for an internal combustion engine including an exhaust purifying filter that can be regenerated by oxidizing particulate matter deposited through filtering of an exhaust gas, utilizing catalytic function, in which fuel is added to an exhaust system from upstream of the exhaust purifying filter, to execute a process of increasing temperature for purifying the particulate matter on the exhaust purifying filter, upon oxidizing the particulate matter.
2. Description of the Background Art
An internal combustion engine, particularly a diesel engine, adopts a technique of arranging an exhaust purifying filter in the exhaust system so that particulate matter contained in the exhaust gas is not discharged to the atmosphere. In such an internal combustion engine, it is necessary to remove the particulate matter deposited on the exhaust purifying filter for regeneration. Therefore, a technique has been known in which, when a certain amount of particulate matter has been deposited, a state of fuel injection is changed (to delayed injection) to increase the temperature of exhaust purifying filter, so as to oxidize and purify the particulate matter.
The fuel supplied by the delayed injection is burned in an expansion stroke, and therefore, full combustion of the entire fuel would not be attained. As a result, the exhaust gas comes to contain HC (carbon hydride) of high concentration. Consequently, sufficient burning cannot be attained at the exhaust purifying filter, either, possibly causing a problem of white smoke.
A technique has been known, in which a glow plug is used for heating a combustion chamber to promote atomization of the fuel, so that the entire fuel supplied by delayed injection is flamed up, whereby discharge of high-concentration HC to the exhaust gas can be prevented (see, for example, Japanese Patent Laying-Open No. 2004-225579, pp. 6-7, FIGS. 2-3).
In an exhaust gas purifier for an internal combustion engine of the type in which fuel is supplied from an addition valve provided in the exhaust system to the exhaust purifying filter and burned for increasing catalyst temperature, it is impossible to directly heat the added fuel by the glow plug and, therefore, atomization of the added fuel cannot be promoted. This may result in problematic emission.
By way of example, dependent on the state of operation of internal combustion engine, an amount of fuel injection might increase abruptly during regeneration control of the exhaust purifying filter. In such a case, the exhaust gas discharged from the combustion chamber has particularly low oxygen concentration. When fuel is added to the exhaust gas of such a low oxygen concentration, HC concentration resulting from the added fuel would be imbalanced to the higher concentration side, relative to the oxygen concentration, than in a normal fuel addition.
As described above, however, atomization of added fuel cannot be promoted by the glow plug, and therefore, sufficient burning of the added fuel is not always possible in the exhaust purifying filter. As a result, HC of high concentration would be discharged to the downstream of exhaust purifying filter, possibly causing the problem of white smoke.